To meet the rapidly increasing demand for a clean electric energy in recent years, electrolytic cells have been utilized in a growing number of fields. Typical examples of such fields are those using various types of batteries such as primary and secondary fuel batteries, and electrolytic industries such as electroplating, NaCl electrolysis and electrolytic synthesis of organic compounds. The electrodes used for these electrolytic cells include those based on electrochemical reactions of electrode itself as an active material as seen in batteries (e.g., lead batteries) and those where the electrode functions as a reaction field to allow progress of electrochemical reactions but the electrode itself does not change. The electrodes of the latter type have been mainly used for new types of secondary batteries and in electrolytic industries. Such new types of secondary batteries have been developed as batteries for storing excess electric power at the nighttime and providing same during the daytime when demand for electric power increases, thereby to level the varying demands, in consideration of ensured effective preservation of electric energy in the future, wherein typically known are zinc-chlorine batteries, zinc-bromine batteries and redox-flow type batteries. These new types of secondary batteries have been developed as back-up power sources of electric power generation system utilizing natural energies such as sunlight, wind power and wave power and as electric power sources for electric automobiles.
Of the new secondary batteries, a redox-flow type battery is superior to other batteries in reliability and economic aspect, and is one of the batteries having the highest likelihood of being put to practical use. This battery consists of an outer tank preserving an electrolyte solution and one or more electrolytic cells, wherein an electrolyte solution containing active materials is supplied from the outer tank to the electrolytic cell to allow electrochemical energy conversion, namely, charge-discharge, on the electrodes incorporated in the electrolytic cell. In general, the electrolytic cell has the liquid flow-through type structure as shown in FIG. 1, since the electrolyte solution is circulated between the outer tank and the electrolytic cell during the charge-discharge. The liquid flow-through type electrolytic cell is called a single cell, which is the minimum unit to be used alone or upon laminating with other single cells. The electrochemical reaction in the liquid flow-through type electrolytic cell is a non-uniform phase reaction on the surface of the electrode which generally accompanies a two-dimensional electrolytic reaction field. A two-dimensional electrolytic reaction field is associated with a difficulty in that the reaction amount per unit volume of the electrolytic cell is small. Then, the reaction amount per unit area, i.e., current density, is increased by a three-dimensionalization of the electrochemical reaction field. FIG. 2 shows a schematic view of the liquid flow-through type electrolytic cell having a three-dimensional electrode.
The electrolytic cell shown in FIG. 2 includes two counter-opposing collectors 1, an ion exchange membrane (separator) 3 between said collectors, and spacers 2 at both sides of the ion exchange membrane 3, which spacers forming flow paths 4a and 4b of electrolyte solution along the collectors 1. An electrode 5 of carbonaceous fiber assembly is installed in at least one of said flow paths 4a and 4b. In this way, a three-dimensional electrode is formed.
In the case of a redox-flow type battery using an acidic aqueous solution of hydrochloric acid containing iron chloride as a positive electrode electrolyte solution and an acidic aqueous solution of hydrochloric acid containing chromium chloride as a negative electrode electrolyte solution, for example, an electrolyte solution containing chromium bivalent ion Cr.sup.2+ is supplied through the liquid flow path 4a on the negative electrode side, and an electrolyte solution containing iron trivalent ion Fe.sup.3 + is supplied through the liquid flow path 4b on the positive electrode side during discharge. In the liquid flow path 4a on the negative electrode side, Cr.sup.2+ releases electrons in the three-dimensional electrode 5 and is oxidized into chromium trivalent ion Cr.sup.3+. The released electrons reduce Fe.sup.3+ to iron bivalent ion Fe.sup.2+ in the three-dimensional electrode 5 on the positive electrode side, after passing through an outer circuit.
The oxidation-reduction during discharge causes insufficient amount of chlorine ion Cl.sup.- in the negative electrode electrolyte solution and excessive Cl.sup.- in the positive electrode electrolyte solution, which imbalance is resolved by the migration of Cl.sup.- through the ion exchange membrane 3 from the positive electrode side to the negative electrode side, thereby achieving a charge balance. Alternatively, hydrogen ion H.sup.+ may migrate through the ion exchange membrane 3 from the negative electrode side to the positive electrode side, thereby also achieving a charge balance. Most iron/chromium redox-flow type batteries use a cation exchange membrane as the ion exchange membrane which balances the charge by the migration of H.sup.+. In addition, the use of an aqueous solution of sulfuric acid containing vanadium, which has a high electromotive force, in recent years has enabled to achieve the high energy density of the battery.
With respect to the liquid flow-through type electrolytic cell, electrodes have been intensively developed, since they are responsible for, for example, the properties of batteries. The electrode which is not an active material itself but functions as a reaction field for promoting the electrochemical reactions of active materials permits application of various materials, in which carbonaceous materials are frequently used in view of electro-conductivity and chemical resistance. In particular, a carbonaceous fiber assembly which is chemically resistant, electro-conductive and liquid-permeating has been used for the electrode of redox-flow type batteries which have been intensively developed for storing electric power.
As the electrode material to be used for such liquid flow-through type batteries, for example, Japanese Patent Unexamined Publication No. 119680/1984 proposes a knit fabric made from carbonaceous fibers. Japanese Patent Unexamined Publication No. 200467/1988 proposes a carbon electrode material of a textile fabric consisting of a thick yarn of No. 5 count or above and a yarn thinner than this in the direction crossing therewith. Japanese Patent Unexamined Publication No. 234612/1994 proposes the use of a nonwoven fabric as the texture.
The liquid flow-through type electrolytic cell having a three-dimensional electrode prepared using such porous electrode material is inevitably subject to liquid flow-through pressure loss due to the electrode material in the electrode. An electrolyte solution is supplied to an electrode with a pump, and a liquid flow-through pressure loss in the electrode causes increase in energy consumption for running a pump to ultimately decrease the total battery energy efficiency. In the case where a porous electrode material comprising a three-dimensional electrode has the same density, the liquid flow-through pressure loss can be decreased by thickening the porous electrode material comprising the three-dimensional electrode, which leads to the reduction of the load on the pump. However, a thicker three-dimensional electrode requires greater amounts of an electrode material to be used, which in turn raises the total production cost of the battery. To avoid such high production cost, Japanese Patent Unexamined Publication No. 200467/1988 proposes a carbon electrode material of a textile fabric consisting of a thick yarn of No. 5 count or above and a yarn thinner than this in the direction crossing therewith. This electrode material gives difficulty in handling as exemplified by slipping or dropping of the thick yarn and/or the thin yarn forming the electrode material, and unachievable cutting into a predetermined size with precision, which is due to the instable shape of the fabric.
Vanadium has been used for a new redox-flow type secondary battery besides Fe/Cr, as mentioned earlier. The oxidative reduction of vanadium ion in the electrode comprising a carbonaceous fiber assembly in said battery is 10 times or more faster than that of chromium ion conventionally used as the negative electrode active material of redox-flow type batteries. The absence of electrolyte solution flow inside the electrode material according to a method called "flow by" wherein an electrolyte solution is flown through the space between a carbonaceous fiber assembly (electrode material) connected to a collector, and a separator results in an electrode reaction which occurs only in the region facing said space. As a result, the battery is insufficient in that the voltage efficiency is low, thus failing to advantageously utilize the accelerated reaction speed.
An attempt to solve this problem by flowing the electrolyte solution inside the fiber assembly (electrode material) increases the pressure loss during the liquid flow, due to the viscosity of the electrolyte solution. In addition, an increase in the flow-through liquid amount to increase the output electric energy per unit time and unit area causes greater pressure loss.